536 Henry C. RicJianh : 



Ilmtnite. — The TiO.^ is extremely high, while Fe.^0.^ is 

 absent, and MgO is present to a small extent, the FeO percent- 

 age being about normal. ♦ 



The mineral is strongly magnetic, which is rather curious, as 

 it is generally stated that only when you get a combination 

 with magnetite does the mineral exhibit strong magnetic pro- 

 perties. The formula for this mineral is RO. ^ROo. 



Determination of P.^O., i.\ the Analyses. 



In determining this constituent by the magnesium-pyrophos- 

 phate method, according to Washington, i the values obtained 

 were obviously high, consequently the method was abandoned, 

 and that of Finkener^ by direct determination of the ammonium 

 phospho-molybdate, adopted with satisfactory results. Dr. H. 

 I Jensen^ has encountered a similar difficulty and overcome it 

 in this manner. 



Calculation of the Mode of the Rock. 



The volume percentage of each mineral in the rock was esti- 

 mated by Rosiwal's'' method. The application of this method 

 to the rock presented considerable difficulty on account of the 

 fine-grained groundmass, which is made up of quartz, felspar 

 and biotite, the average diameter of these grains being only 

 .01 mm., and that of the j^henocrysts .1 mm. 



in the groundmass the biotite was easily distinguished by its 

 colour, while the slight difference in refractive index between 

 the quartz and felspar was utilised in discriminating between 

 these two minerals. 



Sections were cut from the same sample of the rock as the 

 analyses were made from, and as a result of traverses in several 

 directions. Out of a total volume of 6389 tlie volumes of the 

 different minerals were as shown in the accompanying table. 

 The percentage volumes were then calculated and multiplied 



1 II. S. Washinjfton, "Chemical Ati.alysisof Rocks." 



2 Treadwell, "Analytical Chemistry," vol. ii., p. 347. 



3 Proc. Linn. Soc, N.S.W., 1907, vol. xxxii., pp. 908, et seq. 



4 Verhandl d k.k. (icol. Reichsanst, 18s)8, pp. 143, et seq. "The Quantitative Classi- 

 fication of iHiifOus Kdcks," p. 204, 1903. .). P. Iddin.us, .loiiinal of (Jeolojjy, vol. xii. (1904), 

 p. 1%U. 



